I just got a 2nd 5970 installed, but guiminer will only let me use two cores for mining. All four cores correctly appear in my device manager, as well as in other tools such as MSI Afterburner. I can get both cards to work individually.

Have you got a monitor plugged into your second card? If not, you at least need a "dummy plug" connected to one of its outputs, otherwise the 2nd card will not show up in your mining client. I'm not exactly sure why this is, but I believe it has something to do with the card powering down when it is not tasked with any graphic output workloads.